Field of the Invention
In communications systems information can be transmitted and received in a variety of forms. The present disclosure applies to communications systems wherein binary messages or data is transmitted between geographically separated transmitter-receiver units. In general, it is desirable to transmit information between the units of the present system which is compatible with units of other systems presently in use. For example, in private line paging systems, of the type manufactured and sold by the assignee, transceivers are utilized which operate in a frequency range of approximately 300 to 3,000 Hz and control signals are transmitted in the frequency spectrum from zero to 300 Hz and above 2,000 Hz. Therefore, if the present system is to be compatible with the private line paging systems it must operate in a frequency range between 300 and 2,000 Hz. The apparatus disclosed herein is a modulator-demodulator constructed to render the present communications system compatible with other communications systems in which the present system may be completely or partially incorporated.